1. Field of the Invention
The invention concerns method and apparatus for fusing two adjacent bony structures such as a bone joint, especially adjacent vertebrae of the spine.
2. Description of Related Art
Subsequent to injury, disease or other degenerative disorder, the disc, a ligamentous cushion between vertebrae, may undergo a painful deterioration. The disc shrinks and flattens out, and the distance between the vertebral bodies begins to collapse. Subsequently, there may be a progressive degeneration leading to mechanical instability, where painful translocations occur between adjacent vertebrae. The movement-induced pain, may be so disabling that in many such cases, the vertebral motion must be eliminated. Thus, rigid fusions may be the only present means to stop the translocations and relieve the pain.
It is generally held that successful fusions demand a contiguous growth of bone to create a solid mass that will unite the movable elements into one unit. Otherwise, the fusion cannot achieve the tasks of pain reduction maintenance of intervertebral height, and immobility of the vertebrae. When fusion bone is first placed, it is soft and movable, having no cohesive strength. Therefore a variety of appliances have been developed that attempt to hold the vertebrae quite still under conditions of normal spinal activity and daily stress. Bone graft material is placed between the vertebrae, the outer or cortical surfaces of which have been removed or deeply scarified so as to promote the ingrowth of the graft into these recipient sites. Thus positioned, the bone graft slowly unites the vertebrae. Such an appliance is not meant to permanently secure immobility of the segments Bone ingrowth is required for this.
Dependency upon such an appliance as the sole stabilizer is ultimately unsuccessful due to the development of a mechanical gap or transition between the bone and the appliance, leading to structural failure of the bone and adjacent connective tissue. Such failure is seen in fractures, erosion and absorption of bone with potential further collapse The pain may also become progressively disabling.
Approximately 150,000 lumbar spinal fusions were performed in the USA during 1987, as reported by the American Hospital Association. There are many methods for intervertebral fusion. The most successful have achieved a success rate of about 90% in random cases. However, several of these techniques, especially those requiring complex appliances, are difficult to master and are hazardous to nerve and vessel structures normally lying close to the involved bones.
From a biomechanical point of view, the most important location of a spinal fusion is at the mechanical center of rotation between the vertebrae. This point is centered within the disc space. Therefore, an interbody fusion is the most rigid and thus the most sought after method among surgeons. Current methods of interbody fusions are, however, the most hazardous of all spinal fusion methods.
Both anterior (transabdominal) and posterior surgical approaches are used for interbody fusions. Typically, a plug, dowel or segment of bone is driven tightly into a cavity carved inside the interbody, intradiscal space. Since there must be a bone-to-bone bridge created during the fusion process, connective tissue and discal tissue must be removed. Deep cuts within the bone must penetrate into the softer, cancellous region to promote bone growth across the space.
Intervertebral fusions using circular bone grafts have been reported in the orthopedic and neurosurgical literature for some years. B. R. Wiltberger in a paper published in Clinical Orthopedics, Vol 35, pp 69-79, 1964, reviewed various methods of intervertebral body fusion using posterior bone dowels driven firmly into a suitably smaller hole between the adjacent vertebrae. Upon doing so the dowel can split or crack or collapse. The stretched bone might also split and it can be compressed by the dowel to the point that it will not grow normally due to collapse of formerly open pores or vascular channels. If this occurs, there may be a late absorption of surrounding bone and the dowel might loosen, with a renewed danger of expulsion. See also a 2-page brochure from Neurological Surgery Associates of Cincinnati, Inc. entitled "Posterior Lumbar Interbody Fusion Made Simple" which shows, after the bone dowel placement, the "(a)pplication of 5 mm dacron suture around spinous processes."
U.S. Pat. No. 4,501,269 (Bagby) describes a surgical procedure for stabilizing the cervical spine of a horse and says that the procedure:
"is applicable to any human or animal joint formed by opposed contiguous bony surfaces which are covered and separated by intervening cartilage and are surrounded by ligaments which resist expansion of the joint. Specific examples of such joints are a spinal joint between adjacent vertebrae or the ankle joint. The process was developed to immediately stabilize the joint and to further promote ultimate bone-to-bone fusion. . . The implanted structure is in the form of a perforated cylindrical bone basket which can be filled with bone fragments produced during the preparation of the joint. These bone fragments provide autogenous tissue to promote bone growth through the basket, as well as around it. PA1 "The process involves the initial steps of surgically accessing the joint and removing intervening cartilage located between the contiguous bony surfaces. A transverse cylindrical opening is then bored across the contiguous bony surfaces. Immediate stabilization is achieved by driving into the cylindrical opening a hollow basket having a rigid perforated cylindrical wall whose outside diameter is slightly greater than the inside diameter of the cylindrical opening. The implanting of the basket spreads the bony surfaces apart in opposition to the resistance to expansion of the joint provided by the surrounding ligaments" (col. 2, lines 26-55). PA1 "Screw threads are placed in the graft with a small, previously sterilized die. The grooves of the thread can be made as deep as required. The vertebral cervical bodies are prepared according to Cloward's technique. After a cylindrical bed has been drilled in the appropriate intervertebral bodies, the graft is screwed into place with instruments especially developed for this purpose" (p. 750). PA1 "When grafts with a diameter of 14 mm were used, we sometimes threaded the receiving bed with a die-stock of 13 mm to facilitate the insertion" (p. 751). PA1 (a) forming between said bony structures a lateral bore with a female thread that penetrates into their cancellous regions, PA1 (b) forming a hollow cylindrical fusion cage to have an external, substantially continuous helical thread (preferably a V-thread) that is perforated in the valley between adjacent turns and can mate with said female thread, PA1 (c) screwing the cage into said threaded bore, and PA1 (d) packing the cage with bone-inducing substance. PA1 (a) drilling a pilot hole laterally between said bone structures, PA1 (b) inserting a pilot rod into the pilot hole, PA1 (c) fitting a hollow drill over the pilot rod, PA1 (d) with the hollow drill, enlarging the pilot hole to form a bore that penetrates into the cortical bone of each of said bony structures, and PA1 (e) tapping a female thread into the wall of said bore with the crown of the thread penetrating into the cancellous portion of each of said bony structures. PA1 (a) cutting away ligaments to expose the site, PA1 (b) spreading the vertebrae apart, PA1 (c) nibbling away as much of each lamina as is necessary to access the site, PA1 (d) drilling a pilot hole laterally between said vertebrae, each of sufficiently small diameter to be self-seeking of the center of the disc space, PA1 (e) inserting a pilot rod into the pilot hole, PA1 (f) sliding over the pilot rod a hollow lamina drill to cut the spinous process and to score the lamina, PA1 (g) drilling to remove the lamina within the score, PA1 (h) fitting into the resulting arcuate opening in the lamina a C-retractor which has a split cylindrical sleeve of the same diameter as the lamina drill, a handle extending from one end toward the upper end of the spine, and spikes at its other end, PA1 (i) forcing at least one of the spikes into each of said adjacent vertebrae to anchor the C-retractor, PA1 (j) reinserting the pilot rod to rest on the bottom of the pilot hole, PA1 (k) sliding a hollow vertebral drill over the pilot rod and inside the sleeve of the C-retractor, PA1 (l) forming with the hollow drill a bore that penetrates into the cortical bone of each of said vertebrae, PA1 (m) removing the hollow drill, the pilot rod, and the cut bone, and PA1 (n) using the C-retractor as a guide, tapping a female thread, the crown of which extends into the cancellous bone of each of the vertebrae. PA1 a hollow cylindrical shaft having a handle at one end and an external thread which is formed at the other end with at least one scallop that exposes a cutting edge, and PA1 a pilot rod that slidably fits into said bore, projects beyond said other end of the hollow shaft, and is formed with a central recess that communicates with the scallop in the hollow shaft and provides a reservoir for detritus removed by said cutting edge, thus permitting the detritus to be carried away by removing the pilot rod from the hollow shaft. The portion of the pilot rod that projects beyond said other end of the hollow shaft preferably is threaded to carry detritus upwardly to the reservoir.
U.S. Pat. No. 2,537,070 (Longfellow) shows in FIG. 2 a "reinforce 7" that is much like Bagby's fusion basket.
Vich, J. Neurosurg., Vol 63, pp 750-753 (1983) describes a means for cervical spine fusion, using an anterior approach, by surgically implanting a cylindrical bone graft.
Vich's FIG. 2 legend points out that a threaded graft dowel has a larger contact surface than a plain dowel and a greater resistance to pressure and sliding. Vich also says:
An additional desirable effect of an intervertebral fusion is the restoration or maintenance of a normal intervertebral spacing. Spreading devices are generally required in order to restore all or a part of the normal intradiscal height, in the process of placing the fusion material or appliance. When the procedure is performed using the commonly employed posterior approach, a variety of spreaders may be placed between various posterior bony elements normally attached to the vertebrae, such as, dorsal spinous processes or laminas. Using such spreaders, a forward tilt or wedging of the discal space occurs, with the posterior aspect of the space becoming more open than the anterior. When a bone graft of any shape is driven into a cavity that is wedged more open posteriorly between two opposing movable vertebrae, there is a strong propensity for the graft to be retropulsed during the postoperative recovery period as a result of to and from movement between the opposing vertebrae. Thus, to aid in the prevention of graft expulsion, it would be desirable to have the cavity either maintain parallelism or be slightly narrower at its most posterior portion. Ventral to this cavity, the stout ligamentous disc annulus remains and prevents ventral migration of the graft into the retroperitoneal space. Further, there is value in restoring the original spinal lordotic curve, as the fusion grows; this requires that the cavity and the interbody fusion element be placed to promote a normal spinal anatomical position, that is, without wedging of the space in either direction.
In U.S. Pat. No. 4,743,256 (Brantigan) pairs of plugs are implanted as struts spanning and maintaining the disc space between adjacent vertebrae. While bone plugs were previously used in the same way, Brantigan employs "rigid plugs of structural material having porous surfaces to facilitate ingrowth of bone tissue" (col. 2, lines 66-68), inserting these into "grooves bridging the cancellous bone of one vertebral body to the cancellous bone of the subjacent vertebral body. . ." (col. 2, lines 1-6). "The plugs are preferably made of an inert metal substrate such as stainless steel ... having a porous coating of metal particles ..." (col. 3, lines 8-14). The plug of FIG. 12 "has bone piercing tangs or points 31" (col. 5, line 61).